Univention Corporate Server 4.4 erratum 233 (bugfix)

This update addresses the following issue:
* Multicontainer Apps now use a subnet managed by the App Center instead of
creating arbitrary networks. This caused problems with firewalls and
existing networks. The network can be configured by the UCR variable
appcenter/docker/compose/network, which defaults to 172.16.1.1/16. This
change takes effect with new App installations.